Acid pickling process



United States Patent This invention provides an improved process for thepickling in acid baths of IHMQEBS.

The pickling of metals and alloys in acid solutions is commonlyundertaken to remove surface rust, scale, oxide layers, etc., to roughenor efchthe" metal, 0 0 polish "chemically the melal There is frequentlyan evolution of mums-s ray from baths used for such treatments due tothe generation of hydrogen or other gas at the metal surface, or theliberation of steam or volatile acids such as hydrochloric. There isalso a considerable loss of heat due to evaporation from the surface ofthese treatment baths when they are operated substantially above roomtemperature.

The evolution of acid fumes, spray and steam results in wastage ofchemicals and heat, and constitutes a hazard to the health of operatorsand leads to corrosion of plant and buildings unless exhaustingequipment is installed.

In this invention the evolution of acid spray and fume, and steam, froma tank, bath or pit, containin a solution of acid is largely eliminatedby floating on the surface of the acid solution a layer of an inertsubstance which is 1i uid at the tem erature of use of the acid andHibstantlally immiscible therewith. The loss of heat by surfaceevaporation of steam is markedly reduced by such a surface layer; this15 particularly useful when the bath is being kept hot but not inconstant use.

The substance may be, for example, a high boiling point parafiin such asliquid paraffin, petroleum jelly, a paraffin wax, an animal or vegetableoilmolin, tallow or castor oil, bitumen, a low molecular weightsynthetic resin, or a low melting point natural resin.

The layer-forming substance must be inert, i.e. stable at thetemperature of the bath against attack by the solution in the bath, byany hydrogen generated, or by atmospheric oxygen. It must, of course,also be less dense than the acidic solution. It is preferable that itcontains very little or no material volatile at the temperature ofoperation of the bath and it should be of such viscosity at theoperating temperature of the bath that the layer will readily reformafter being broken by, for example, the entry or exit of articles to thebath, or the exit of bubbles of gas or vapour.

The preferred layer-forming substances are, at the temperature ofoperation of the bath, liquid hydrocarbon materials such as variousgrades of oils, the viscosity being dependent largely on the operatingtemperature of the bath. It has been found that when pickled articlesare withdrawn from the pickling bath through a layer of such liquidhydrocarbon material the hydrocarbon shows little tendency to cling tothe article and is readily washed off. Of the liquid hydrocarbonmaterials those which are chemically saturated (i.e. contain nounsaturated carbon to carbon bonds) are preferred due to their highdegree of stability.

The inert surface layer is effective in substantially reducing evolutionof acid fumes from the bath. The following table shows the time requiredto decolourise a piece of alkaline phenol phthalein paper placed above asolution of 20% HCl at 95 C. covered to various depths with layers of ahydrocarbon oil having a viscosity of 8 Patented June 6, 1961centipoises at 95 C. A piece of the same paper was decolourised in 5minutes by a normal atmosphere:

Depth of layer, mm 0 2.0 4.0 6.0 10

Time/secs 4 50 65 70 Other experiments were carried out to determine theloss of hydrochloric acid from a 50% acid bath at C. covered with layersof the same low viscosity oil to various depths.

Depth of oil layer,

Loss of h drochlorlc acl Ens/sq. metre/ The loss from the uncovered bathat room temperature was 3.3 gms./sq. metre/hour.

It is preferable for most effective reduction of heat losses and acidspray that the layer of substance should be at least 10 mm. thick.

in practice it is found that the depth of the layer can be variedbetween fairly wide limits, these limits being dependent on theviscosity range of the materials used, the temperature of operation ofthe bath and the amount of foam formed at the interface between theinert substance and the aqueous acid.

The viscosity of the layer substance at the operating temperature canvary widely but it should preferably not exceed 100 poises. To ensurerapid removal of the layer forming substance from the metal article andto reduce drag out of the substance to a minimum the viscosity of thesubstance at the temperature of operation of the bath may be as low as0.5-2 centipoises.

Although in general operating viscosities less than 100 centipoises arepreferred, care should, of course, be taken that the flash point of thelayer forming substance is above that of the temperature of operation.For this reason a viscosity range of from 2-10 centipoises at theoperating temperature is preferred.

A surprising feature of the invention is that the tendency of thesurface layer to cling to articles entering the bath does not seriouslyaffect the speed at which the acid reacts with the metal. in any casethis tendency may be minimised by using a layer of a substance with alow viscosity at the temperature of operation. Removal of adherentsurface layer from the articles being treated may also be assisted bythe addition of surface active agents to the acid solution. It ispreferred that the surface tension of the acid solution be reduced inthis way to less than 40 dynes/cm., but the quantity of surface activeagent added should be less than that required to disperse or emulsifythe surface layer into the acid solution.

Further, after treating metals in acid solution it is customary to rinsethem in water to remove residues of acid and acidic salts, and for thispurpose a rinse of cold running water is often used. when u acidsolution covered by a surface layer as described in this inveit'ibn theyretain, on removal from the solution,

rinse. Where it is desired to free the metal entirely from the surfacelayer substances it is best to employ an overflowing rinse and leave themetal immersed until the surface of the rinse is clear of surface layersubstances. A simple gravity type oil separator may be used to avoidwasting the surface layer substance removed in this way.

Since the inert layers of the present invention are so effective inreducing fuming from hot acid baths it is desirable to colour them, forexample by adding a red dye, in order to warn operators that the bathsare, in fact, hot.

The invention is illustrated by the following examples:

Example I In the pickling treatment of rusty iron in 20% hydrochloricacid solution (200 gm. HCl gas/litre) in water at 90 C., a process whichnormally has to be carried out at a maximum temperature of 35 C., theloss of acid from a bath without surface layer was found to be 1550 gm./sq. metre/hour.

The bath was then covered with a 6 mm. thick layer of a hydrocarbonmaterial having a viscosity of 3.5 centipoises at 90 C. and a flashpoint of 155 C. On continuing the pickling treatment under this surfacelayer the loss of acid was less than 10 gm./sq. metre/hour.

The pickling treatment was carried out in a 5 litre glass beaker lightlylagged with asbestos string. 308 watts of electricity were required tomaintain a steady temperature of 90 C. without a surface layer. With thesurface layer only 89 watts of electricity were required to maintain 90C.

Example II A bath was made up to the following composition:

' Percent by weight Nitric acid (S.G. 1.42.14 11.0 W 40% HF) 5.0Non-ionic surface active agent 1 Water Remainder The surface activeagent was an anhydrous condensation p wam rmm WW slightly soluble in theinert layer escri e ow, and reduced the surface tension of the bath to36 dynes/cm.

This bath was covered with a layer of saturated hydrocarbon material 12mm. thick and operated at 24 C. The hydrocarbon material was an oilhaving a viscosity of 5.5 centipoises at the operating temperature andhad a flash point of 125 C.

Titanium panels were treated in this bath for minutes followed byrinsing in water and drying. All scale had been removed withoutnoticeable evolution of acid spray or fumes from the surface of thebath, though the passage of small quantities of gaseous nitrogen oxidesthrough the layer was, of course, not prevented.

Example III A bath was made up to the following composition:

The surface active agent was an akyl trimethyl, ammonium salt whichreduced the surface tension of the bath to 30 dynes/cm.

This bath was covered with an inert layer as in Exam- 70 ple II andheated to 60 C. Stainless steel wire was treated in this bath for 5minutes followed by rinsing in water and drying. All scale had beenremoved without noticeable evolution of spray or fumes from the surfaceof the bath.

Example IV A bath was made up to the following composition:

Percent by weight Sulphuric acid (S.G. 1.84) 15.0 Inhibitor 0.005Anionic surface active agent 0.1 Oil soluble red dye 0.01 WaterRemainder The surface active agent was a secondary alkyl sulphate whichreduced the surface tension of the bath to 33 dynes/cm. The inhibitorwas based on di-ortho tolyl thio urea.

This bath was covered with a layer of hydrocarbon material 12 mm. thickand heated to 65 C. The bydrocarbon material had a viscosity of 5.5centipoises at the operating temperature and a flash point of 155 C.Scaly steel panels were treated in this bath for 30 minutes followed byrinsing in water and drying. All scale had been removed withoutnoticeable evolution of spray or fume. The trace of hydrocarbon materialcarried over on the panels was removed without difliculty in the waterrinse.

Example V A bath was made up to the following composition:

Percent by weight Hydrochloric acid (S.G. 1.916) 26.0 Inhibitor 0.1Surface active agent (as in Example H) 0.1 Water Remainder A bath wasmade up to the following composition:

Percent by weight Sulphuric acid (S.G. 1.84) 10.0 Inhibitor 0.005 WaterRemainder The inhibitor was based on di-ortho tolyl thio urea.

A layer of castor oil (S.G. 0.962-0.966 at 15 C., viscosity 700centipoises at 25 C.) 12 mm. thick was poured over the bath. Rusty ironbolts were treated in this bath at 70 C. for 30 minutes. The bolts werewithdrawn from the bath through the oil layer and rinsed. After rinsingthey still showed water break and the oil layer remaining wassatisfactory at a temporary protective coating for the rust free bolts.

There is an advantage in having a surface active agent present in thesurface layer to facilitate the removal of any adherent surface layerwhen subsequently rinsing in water. It is an additional advantage toemploy a surface active agent which is soluble both in the acid bath andin the surface layer so that the one surface active agent fulfils bothfunctions. The surface layer can then act as a reservoir to take up anyexcess of surface active agent which may throw out of solution in thebath at elevated temperatures. The extent to which the surface activeagent is soluble in the bath and layer respectively at the operatingtemperature is important. Alkyl trimethyl ammonium salts have adequatewater solubility up to C. to prevent the bath being denuded by solutioninto the layer. Some agents may be insufficiently water soluble and bepreferentially dissolved by the layer which then adheres to work beingprocessed and may become emulsified into the treatment bath.

What I claim is:

1. A process of pickling metal which comprises providing an acidpickling solution, covering all of the surface of said solution with anon-aqueous surface layer of inert liquid which is substantiallyimmiscible with said pickling solution and substantially nonvolatile andstable at the operating temperature of said solution, including in saidsurface layer and said pickling solution a surface active agent which issoluble in both said layer and said pickling solution, thereafterpassing the metal to be pickled through said surface layer and into saidsolution, withdrawing the thus pickled metal through said surface layer,and then rinsing said metal with water whereby the surface active agentin said surface layer facilitates the removal of any adherent surfacelayer.

2. A process of pickling metal which comprises providing an acidpickling solution, covering all of the surface of said solution with anon-aqueous surface layer of saturated hydrocarbon which is liquid atthe temperature of said solution and substantially immiscible therewith,said liquid being stable and substantially non-volatile at the operatingtemperature of said solution and inert with respect to the constituentsof said solution, said surface layer having a viscosity of from 2-10centipoises and a minimum thickness of at least mm., including in saidsurface layer and said solution a surface active agent which is solublein both said layer and solution, the amount of surface active agentdissolved in the solution being sufficient to lower the surface tensionto less than dynes/cm., said surface active agent having suflicientsolubility in said surface layer to take up any surface active agentseparated out from said pickling solution, thereafter passing the metalto be pickled through said surface layer and into said solution,withdrawing the thus pickled metal through said surface layer, and thenrinsing said metal with water whereby the surface active agent in saidsurface layer facilitates the removal of any adherent surface layer.

References Cited in the file of this patent UNITED STATES PATENTS870,508 Hanna et a1. 3911 .4- Nov. 5, 1907 1,329,467 Miskella "4311329.!Feb. 3, 1920 1,817,527 Schlotter "49.111221- Aug. 4, 1931 2,115,005Blaut Apr. 26, 1938 2,524,825 Pabst .!f L' 3 Oct. 10, 1950 2,583,165Campbell z1t Jan. 22, 1952 2,640,767 Easley et a1. ze!z June 2, 1953OTHER REFERENCES Voznesenski et al.: Chemical Abstract, vol. 37, 001.

1. A PROCESS OF PICKLING METAL WHICH COMPRISES PROVIDING AN ACIDPICKLING SOLUTION, COVERING ALL OF THE SURFACE OF SAID SOLUTION WITH ANON-AQUEOUS SURFACE LAYER OF INERT LIQUID WHICH IS SUBSTANTIALLYIMMISCIBLE WITH SAID PICKLING SOLUTION AND SUBSTANTIALLY NON-VOLATILEAND STABLE AT THE OPERATING TEMPERATURE OF SAID SOLUTION, INCLUDING INSAID SURFACE LAYER AND SAID PICKLING SOLUTION A SURFACE ACTIVE AGENTWHICH IS SOLUBLE IN BOTH SAID LAYER AND SAID PICKLING SOLUTION,THEREAFTER PASSING THE METAL TO BE PICKLED THROUGH SAID SURFACE LAYERAND INTO SAID SOLUTION, WITHDRAWING THE THUS PICKLED METAL THROUGH SAIDSURFACE LAYER, AND THEN RINSING SAID METAL WITH WATER WHEREBY THESURFACE ACTIVE AGENT IN SAID SURFACE LAYER FACILITATES THE REMOVAL OFANY ADHERENT SURFACE LAYER.